1. Field of the Invention
A quantum cascade laser (QCL) is a unipolar semiconductor device that can be readily engineered to emit over a variety of wavelengths, including but not limited to, the mid-infrared and terahertz portions of the electromagnetic spectrum. Device growth and processing can be based on established techniques and widely available materials, such as InP and GaAs and other III-V semiconductor materials. The present disclosure relates to quantum cascade semiconductor lasers (QCLs) and, more particularly, to methods of fabricating QCLs and the corresponding QCL structures.
2. Technical Background
The present inventors have recognized that, in semiconductor lasers that utilize interband lasing transitions, the quantum wells and barrier layers that compose the active region are often sandwiched between n-type and p-type layers on opposite sides of the active region, with the p-type layers typically above the active region. These p-type layers are typically not very conductive. Therefore, interrupting the electrical contact layer or metal on top of the p-doped layer(s) typically provides sufficient electrical isolation between distinct regions of the laser structure. In contrast, the present inventors have recognized that a QCL is a unipolar device where the layers both below and above the active core are of the same conductivity type, typically n-type, and that n-type layers are highly conductive. Accordingly, electron diffusion from one area to an adjacent area above the active core cannot be prevented simply by interrupting the electrical contact layer or metal between sections of the n-type layer to be electrically isolated.
Although the methodology of the present disclosure has applicability to a variety of semiconductor laser configurations, the present inventors have recognized that the need for effective isolation is particularly acute in the context of distributed Bragg reflector (DBR) QCLs, which comprise an active region, a wavelength selective region, and, optionally, a phase region.